1. Field of the Invention
The present invention relates to coupling tubulars, and in particular to coupling expandable tubulars, most particularly expandable downhole tubulars.
2. Description of the Related Art
Downhole tubulars, such as bore-lining casing and liners, are typically coupled together by means of threaded connectors, or by providing adjacent ends of tubulars with male and female, or pin and box, threaded ends. For conventional applications, such coupling arrangements are generally satisfactory, providing secure, pressure tight connections. However, where strings of tubulars are to be expanded to describe a larger diameter, it has been found that the integrity of the coupling may be compromised.
This is particularly the case where expansion is achieved using a rotary expansion tool. Such a tool expands the male or pin thread portion by cold working, reducing the wall thickness of the portion resulting in a corresponding increase in circumference and diameter of the portion, which tends to be accompanied by axial elongation of the threaded portion. However, the female or box thread portion is expanded by contact with the expanding male or pin thread portion. This tends to induce axial contraction of the female thread portion. The applicant has found that in a conventional thread this differential expansion tends to produce an adverse effect on the thread integrity. In particular, it has been found that the axial contraction or shrinkage of the box portion is greater than the elongation of the pin portion, and causes the wall sections over the roots of the box portion to neck giving the outer surface of the box portion a rippled appearance. In extreme cases the differential expansion is sufficient to part the connection.
Integrity problems may also occur in couplings which are expanded using alternative expansion methods such as solid cone expansion or hydraulic expansion techniques, which may also cause differential deformation in the different thread parts of the connection. For example, expansion of a tubular using a cone expander may cause the tubular to axially contract as it is radially expanded. However, when the expansion cone is passed through a conventional threaded connection, the male and female, or pin and box thread portions are caused to axially contract at different rates, thus causing adverse deformation due to the interference between the different thread portions. It is this adverse deformation which diminishes the mechanical and hydraulic integrity of the connection.
Specifically, applicant has observed that during solid cone expansion of a conventional connection, the box is initially caused to move or pull away axially with respect to the pin causing interference between adjacent threads resulting in deformation of inter-engaging thread portions, with further expansion causing differential axial shrinkage in both the pin and box, the overall result being an adverse interference in the connection.
It is among objectives of embodiments of the present invention to provide coupling arrangements for tubulars which will seek to retain mechanical and pressure integrity following expansion of the tubulars.